The knee joint is one of the strongest joints in the body because of the powerful ligaments that bind the femur and tibia together. Although the structure of the knee provides one of the strongest joints of the body, the knee is usually one of the most frequently injured joints, e.g., athletes frequently stress and tear knee ligaments. The large number of ligament injuries has given rise to considerable innovative surgical procedures and devices for replacing and reconstructing torn or dislocated ligaments, typically involving grafting autografts, allografts, or a synthetic construct, to the site of a torn or dislocated ligament. For example, the replacement of an anterior cruciate ligament (ACL) may involve transplanting a portion of the patellar tendon, looped together portions of semitendinosus-gracilis (hamstring) tendons, or donor achilles tendons, to attachment sites in the region of the knee joint.
The most widely used technique for the reconstruction of the ACL is known as the Jones procedure. The basic steps in the procedure include: harvesting a graft made from a portion of the patellar tendon with attached bone blocks; preparing the graft attachment site (e.g., drilling holes in opposing bones of the joint in which the graft will be placed); placing the graft in the graft attachment site; and rigidly fixing the bone blocks in place within the graft site, i.e., the holes or “bone tunnels”. The screws used to fix the graft in place are called “interference screws” because they are wedged between the bone block and the wall of the hole into which the bone block fits. Typically, there is very little space between the bone block and the hole in the bone at the fixation site.
Interference screws for anchoring ligaments to bone are typically fabricated from medically approved metallic materials that are not naturally absorbed by the body. A disadvantage of such screws is that once healing is complete, an additional surgical procedure may be required to remove the screw from the patient. Metallic screws may include a threaded shank joined to an enlarged head having a transverse slot or hexagonal socket formed therein to engage, respectively, a similarly configured, single blade or hexagonal rotatable driver for turning the screw into the bone. The enlarged heads on such screws can protrude from the bone tunnel and can cause chronic irritation and inflammation of surrounding body tissue.
Permanent metallic medical screws in movable joints can, in certain instances, cause abrading of ligaments during normal motion of the joint. Screws occasionally back out after insertion, protruding into surrounding tissue and causing discomfort. Furthermore, permanent metallic screws and fixation devices may shield the bone from beneficial stresses after healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps to prevent decalcification of the bone. Under some conditions, the stress shielding which results from the long term use of metal bone fixation devices can lead to osteoporosis.
Biodegradable or bioabsorbable interference screws have been proposed to avoid the necessity of surgical removal after healing. Because the degradation of a biodegradable screw occurs over a period of time, support load is transferred gradually to the bone as it heals. This reduces potential stress shielding effects. Conventional bioabsorbable interference screws are softer and weaker than metallic compositions, such that they are not self-tapping, requiring the holes drilled into the bone to be tapped. The necessity to tap holes in the injured bone adds to the complexity of the surgical procedure and lengthens the time required to complete the operation.
Considerable effort has been expended to increase the stiffness and strength of bioabsorbable materials through various composite technologies, such as incorporating strong, stiff, non-absorbable, inorganic structural fibers or particles made from carbon or glass, as reinforcing agents in a bioabsorbable polymeric matrix. The disadvantage of this approach is that the non-absorbable fibers remain in the body tissue after the bioabsorbable polymer has been absorbed and may migrate or cause tissue inflammation. Composite bioabsorbable screws may also be prepared by incorporating inorganic, bioabsorbable glass or ceramic reinforcement fibers or particles in a bioabsorbable polymer matrix. However, lack of reinforcement-to-matrix interfacial bonding leads to poor load transfer between the reinforcement and the matrix. The weakness of the interface is accentuated when the implants are placed in the human body and may result in compromised long-term performance.
Reinforced bioabsorbable composite screws have also been made by adding an organic bioabsorbable reinforcing fiber to a bioabsorbable polymer matrix. Similarly, highly drawn fibers of polylactide (PLA) or polyglycolide (PGA) can be fused to form a bioabsorbable polymeric screw with increased stiffness and strength. Unfortunately, the consolidation or the melting temperature of the matrix usually causes the biocompatible organic fibers to partially relax their molecular orientation, thereby losing their strength and stiffness and adversely affecting the properties of the composite. Thus the efforts to utilize bioabsorbable materials for orthopedic load bearing applications has not been entirely successful.
Accordingly, there is a need for interference screws composed mainly of bioabsorbable materials that do not require tapped holes for insertion into bone.